Missing Montana mother and teacher Sherry Arnold is dead, according to a post on her school district's website. The FBI announced today that they have one person in custody and another being questioned.

Arnold, 43, disappeared around 6:30 a.m. on Jan. 7 when she went out for an early morning run near her home in Sidney, Mont. A single running shoe was the only trace of Arnold left behind.

"Based on a tip from the public, law enforcement took one adult male into custody, and another adult male is being questioned by police," FBI spokeswoman Debbie Dujanovic Bertram said in a statement. "The investigation to determine what happened to Ms. Arnold continues."

Sidney Public School officials posted a message on the school district's website saying that they were informed of her death this morning, after Arnold's family was informed. Arnold was a married mother of two, and stepmother of three.

The school district will release students from school at 2 p.m. and "additional counselors are on hand this morning in all buildings and parents, if needed, may check their students out from each building."

Arnold was a teacher in Sidney since 1993, teaching at both the middle school and high school, where she was a math teacher at the time of her death.

"Sherry is one of those teachers that every parent wants in front of their child," Sidney School District Superintendent Daniel Farr told ABCNews.com.

"She is caring, first and foremost, one that is there before and after school helping students," Farr said. "She gets to know her students. She becomes a mentor to many a student that goes on to become a math teacher."

Search and rescue teams used canine units, private planes, a helicopter, searchers on ATVs and on horseback. Hundreds of community members turned out to help the Sidney Police Department and the FBI search for Arnold.

Sidney is a small town of roughly 5,000 people that has experienced a sudden influx of out-of-towner oil workers following the Bakken oil boom in North Dakota, according to the Billings Gazette.

Sidney Police Chief Frank DiFonzo told the newspaper that crime rates have gone up since the influx in areas including bar fights, domestic violence and drunk driving.